The present invention relates to a cryogen delivery apparatus and method for regulating the cooling potential of a flowing cryogen. More particularly the present invention relates to a cryogen delivery apparatus and method in which the flowing cryogen is delivered as a two phase flow containing gaseous and liquid forms of the cryogen and the cooling potential of the flowing cryogen is regulated by regulating proportions of the gaseous and liquid forms of the cryogen contained within the two phase flow
The gaseous and liquid forms of nitrogen are utilized in the blow molding of plastic articles. In blow molding, a cylinder of semi-molten plastic, called a parison, is extruded so that it descends by gravity into position between a pair of opposed mold sections. In one type blow molding process, gaseous nitrogen is released into the parison through a blowing pin until the plastic fits the mold. The gaseous nitrogen is produced by allowing liquid nitrogen from a liquid supply tank to absorb heat in a pipe line leading to the blowing pin.
During the blowing cycle, the injection system gradually cools until liquid nitrogen enters the mold in a fine atomized spray to cool the molded article. In another type of blow molding process air is released into the parison until the plastic fits the mold. Thereafter, liquid nitrogen is injected through the blowing pin to cool the molded article. After the mold is cooled, the mold sections are spread apart for removal of the molded plastic article.
In other cryogenic applications, it is necessary to only deliver measured amounts of a liquid cryogen. For instance, measured amounts of liquid nitrogen are delivered to food containers for producing an inerting atmosphere. In another application, measured amounts of liquid nitrogen are delivered to food containers so that when sealed, the interior of the container is pressurized as the liquid nitrogen boils off within the container. Such pressurization enables the container to maintain its structural integrity.
In all of the above-described applications, which it should be pointed out are described in relation to nitrogen for exemplary purposes only, it is necessary to repeatedly deliver exact amounts of pure liquid and/or gaseous forms of nitrogen. In case of delivery of measured amounts of a liquid cryogen, such as liquid nitrogen in the food process industry, the liquid cryogen is metered by valves, which in the cryogenic environment tend to wear out rather rapidly. Moreover, in the injection blow molding art, the temperature of the liquid nitrogen in the storage tank varies after each filling of storage tank and therefore, the quality of liquid nitrogen that is delivered is also variable.
The present invention solves these problems by providing an apparatus that can repeatedly and intermittently deliver measured amounts of a cryogen in either a liquid and/or a gaseous form, and which does not utilize conventional valves for the metering of the liquid form of the cryogen.
A further problem exists in controlling or metering the exact amount of cooling potential supplied by a cryogen. For instance, in the blow molding art, too much liquid nitrogen may be supplied. In such case, the liquid nitrogen pools in the plastic article and is thus, wasted. Moreover, such pooling also produces uneven cooling of the molded article which can result in discoloration and unacceptable deformities in the finished molded article.
The present invention solves this latter problem by providing an apparatus and method in which a flowing cryogen is delivered with a regulated cooling potential. The regulation of the cooling potential allows the cryogen usage in a particular cryogenic cooling application to be optimized so that the cryogen is not wasted.